Battery case lid and manufacturing method for battery case lid

ABSTRACT

It is an objective of the present invention to provide a battery case lid and a manufacturing method for the battery case lid which inhibit work hardening of a metal plate workpiece and which facilitate manufacture of a battery case lid. Provided is a battery case lid ( 1 ) formed by working a metal plate, including: a substrate section ( 2 ) and an explosion-proof valve ( 4 ) formed in the substrate section ( 2 ), wherein the explosion-proof valve ( 4 ) has a reduced thickness section ( 41 ) thinner than the substrate section ( 2 ), and the reduced thickness section ( 41 ) is formed by extending the metal plate by applying pressure while the metal plate is kept unrestrained.

TECHNICAL FIELD

The present invention relates to a battery case lid for use in asecondary battery.

BACKGROUND ART

Secondary batteries comprise, for example, a battery case containing anelectrolyte and a battery case lid closing an opening of the batterycase. The battery case lid mainly includes a substrate section welded tothe battery case and an explosion-proof valve for releasing the internalpressure from inside the battery case when abnormality occurs to theelectrolyte. The explosion-proof valve is thinned compared to thesubstrate section and designed to rupture when the internal pressureexceeds a predetermined value.

Conventionally, in a case where the explosion-proof valve is formed, theexplosion-proof valve is joined to the substrate section by, forexample, welding. When the explosion-proof valve is joined to thesubstrate section by welding, however, it is difficult to make thethickness of the joining portion uniform. This gives rise to a problemthat an operational pressure at which the internal pressure is releasedcannot be made constant. In addition, there is another problem that thewelding involves complicated joining operation.

Patent document 1 discloses a method of forming an explosion-proof valveby press-working a single metal plate. In the method of Patent document1, a single metal plate is coined to form a reduced thickness sectionthinner than a substrate section, and then, a groove is formed in thereduced thickness section along the periphery thereof. A portion of thereduced thickness section where the groove is formed is even furtherreduced in thickness compared to the remainder of the reduced thicknesssection in order to ensure the rupture of the explosion-proof valve whenthe internal pressure exceeds a predetermined value. In addition, in themethod of Patent document 1, after the groove is formed, annealing isperformed so that the operational pressure of the work-hardenedexplosion-proof valve can be controlled.

PRIOR ART DOCUMENT Patent Document

-   Patent document 1: Japanese Patent No. 3222418

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

When the reduced thickness section is formed by press-working the metalplate through a process such as coining, and then the groove is formedin the reduced thickness section, however, there has been a problem thatthe metal plate workpiece (work), which is work-hardened, is prone tocrack in the groove formation process. Especially in recent years, therehas been another problem that conventional manufacturing methods cannotrespond to the needs of the downsizing and the reduction in thickness ofsecondary batteries. Further, in the method of Patent document 1, themetal plate workpiece is work-hardened and thus needs to be annealed tobe softened so that the operational pressure of the explosion-proofvalve can be controlled. Accordingly, the method of Patent document 1has a problem of complicated manufacturing process. In addition, theexplosion-proof valve, which is reduced in thickness, needs to beprotected from external force, heat, and the like.

The present invention has been made from the above view point and anobjective of the present invention is to provide a battery case lidwhich inhibits work hardening of a metal plate workpiece and which iseasy to manufacture and a manufacturing method for the battery case lid.Another objective of the present invention is to provide a battery caselid which is easy to manufacture and which enables protection of theperiphery of an explosion-proof valve.

Means to Solve the Problems

The present invention has been made to solve the above problems, and itis an objective of a first aspect of the present invention to provide abattery case lid formed by working a metal plate, the battery case lidcomprising: a substrate section; and an explosion-proof valve formed inthe substrate section, wherein the explosion-proof valve has a reducedthickness section thinner than the substrate section, and the reducedthickness section is formed by extending the metal plate by applyingpressure with the metal plate kept unrestrained.

According to the above configuration, since the metal plate is pushed inan unrestrained state, a reduction is made in an amount of strainsintroduced at the formation of the reduced thickness section, and as aresult, work hardening of the metal plate workpiece is inhibited. Thisprevents crack of the reduced thickness section at the formation of, forexample, a groove in the reduced thickness section. Moreover, since workhardening of the metal plate workpiece is inhibited, the annealingprocess may be omitted, reducing the number of operational steps.

Further, preferably, the reduced thickness section has a groove.According to the above configuration, since a portion where the grooveis formed is even further reduced in thickness, the rupture of thegroove is ensured when the internal pressure exceeds a predeterminedvalue.

Further, preferably, the explosion-proof valve is surrounded by acylindrical wall portion thicker than the reduced thickness section.Moreover, preferably, the explosion-proof valve is surrounded by afolded part thicker than the reduced thickness section, the folded partbeing formed by folding the metal plate. According to the aboveconfiguration, the reduced thickness section can be protected.

It is an objective of a second aspect of the present invention toprovide a manufacturing method for a battery case lid including anexplosion-proof valve, the manufacturing method comprising: an extendingstep of pushing an extending punch from a first side of a metal platedisposed over an extending die having a recess, to form a reducedthickness section constituting the explosion-proof valve, wherein therecess has an inter diameter greater than an outer diameter of theextending punch, and wherein in the extending step, the extending punchis pushed while the metal plate is kept unrestrained, and the metalplate is pressed to be extended between a bottom face of the recess anda pressing face of the extending punch, to form the reduced thicknesssection.

According to the above manufacturing method, since the metal plate ispushed by the extending punch while being kept unrestrained, a reductionis made in an amount of strains introduced at the formation of thereduced thickness section, and as a result, work hardening of the metalplate workpiece is inhibited. This prevents crack of the reducedthickness section at the formation of, for example, a groove in thereduced thickness section. Moreover, since work hardening of the metalplate workpiece is inhibited, the annealing process may be omitted,reducing the number of operational steps.

Further, preferably, the above manufacturing method further comprises agroove forming step of forming a groove in the reduced thicknesssection. According to the above configuration, since a portion where thegroove is formed is even further reduced in thickness, the rupture ofthe groove is ensured when the internal pressure exceeds a predeterminedvalue.

Further, preferably, in the extending step, a cylindrical wall portionis formed around the reduced thickness section, the cylindrical wallportion having a height greater than a depth of the recess of theextending die. According to the above manufacturing method, a reductionis made in an amount of strains introduced at the formation of thereduced thickness section.

Further, preferably, the extending step is followed by a bending step inwhich the reduced thickness section is pressed by a bending punchsmaller in outer diameter than the reduced thickness section from asecond side of the metal plate, to bend a cylindrical wall portionformed around the reduced thickness section.

According to the above configuration, since the height of thecylindrical wall portion can be reduced, the height of the battery caselid can be reduced.

Further, preferably, the bending step is followed by a folding step inwhich a bent portion formed by bending the cylindrical wall portion inthe bending step is pushed by a folding punch from the second side ofthe metal plate, to fold the bent portion.

According to the above configuration, since the height of thecylindrical wall portion can be reduced, the height of the battery caselid can be reduced. Moreover, since the explosion-proof valve issurrounded by the part formed thick by folding the metal plate, theperipheral area of the explosion-proof valve can be protected.

Further, preferably, a clearance between a side face of the recess ofthe extending die and the extending punch is set to be smaller than athickness of the metal plate.

If the clearance between the side face of the recess and the extendingpunch is equal to or greater than the thickness of the metal plate,there are cases where strains (creases) occur in a portion of the metalplate workpiece which is not sandwiched between the extending die andthe extending punch during the operation of extending the metal plateworkpiece by applying pressure to form the reduced thickness section, anoperation subsequent to drawing the metal plate workpiece into theextending die.

According to the above configuration, the clearance c between the sideface of the recess and the extending punch is set to be smaller than thethickness of the metal plate. Because a portion of the metal plateworkpiece that first starts moving in the extending step is a portionthereof sandwiched between the extending die and the extending punch,the above configuration can serve to reduce strains in a portion of themetal plate workpiece which is not sandwiched between the extending dieand the extending punch, and consequently improves the quality of theproduct.

Further, preferably, the pressing face of the extending punch has aspherical face which is convex in a pressing direction of the extendingpunch.

According to the above configuration, since the metal plate workpiece ispressed to be extended radially from the center of the metal plateworkpiece, the reduced thickness section can have a uniform thickness.

Further, preferably, a side face of the extending punch has a topsection with a lateral protrusion formed thereon.

According to the above manufacturing method, friction between the sideface of the extending punch and the metal plate workpiece at the workingcan be avoided. This prevents the metal plate workpiece from beingdeformed due to friction resistance.

Further, preferably, the extending step is followed by a correcting stepin which the reduced thickness section is corrected to be flat. Inperforming the extending step, there are cases where the reducedthickness section becomes slightly convex in the pushing direction ofthe punch. According to the above configuration, the reduced thicknesssection can be flattened.

It is an objective of a third aspect of the present invention to providea battery case lid formed by working a metal plate, the battery case lidcomprising: a substrate section; and an explosion-proof valve formed inthe substrate section, wherein the explosion-proof valve has a reducedthickness section thinner than the substrate section, and theexplosion-proof valve is surrounded by a cylindrical wall portionthicker than the reduced thickness section.

It is an objective of a fourth aspect of the present invention toprovide a battery case lid formed by working a metal plate, the batterycase lid comprising: a substrate section; and an explosion-proof valveformed in the substrate section, wherein the explosion-proof valve has areduced thickness section thinner than the substrate section, and theexplosion-proof valve is surrounded by a folded part thicker than thereduced thickness section, the folded part being formed by folding themetal plate.

According to the above configuration, the reduced thickness section canbe protected.

Further, preferably, the reduced thickness section has a groove.According to the above configuration, since a portion where the grooveis formed is even further reduced in thickness, the rupture of thegroove is ensured when the internal pressure exceeds a predeterminedvalue.

Effect of the Invention

The battery case lid and the manufacturing method for the battery caselid according to the present invention inhibit work hardening of a metalplate workpiece and facilitate manufacture of a battery case lid.Further, the battery case lid according to the present invention enablesprotection of the periphery of an explosion-proof valve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a battery case and a battery case lidaccording to the present embodiment.

FIG. 2A is a plan view of the battery case lid according to the presentembodiment, and FIG. 2B is a cross sectional view thereof.

FIG. 3 is an enlarged cross sectional view showing a first groove and asecond groove according to the present embodiment.

FIG. 4 is a flowchart of a manufacturing method according to the presentembodiment.

FIGS. 5A-5D are cross sectional views showing step by step an extendingstep according to the present embodiment.

FIG. 6 is a cross sectional view of a metal plate workpiece after theextending step according to the present embodiment.

FIGS. 7A and 7B are respectively cross sectional views of the metalplate workpiece before and after a first correcting step according tothe present embodiment.

FIGS. 8A and 8B are respectively cross sectional views of the metalplate workpiece before and after a second correcting step according tothe present embodiment.

FIGS. 9A and 9B are respectively cross sectional views of the metalplate workpiece before and after a bending step according to the presentembodiment.

FIGS. 10A and 10B are respectively cross sectional views of the metalplate workpiece before and after a preliminary folding step according tothe present embodiment.

FIGS. 11A and 11B are respectively cross sectional views of the metalplate workpiece before and after a primary folding step according to thepresent embodiment.

FIGS. 12A and 12B are respectively cross sectional views of the metalplate workpiece before and after a groove forming step according to thepresent embodiment.

FIG. 13 is an enlarged cross sectional view of a modification of thegrooves of the explosion-proof valve.

FIG. 14 is a cross sectional view of a first modification of the batterycase lid.

FIG. 15 is a cross sectional view of a second modification of thebattery case lid.

FIG. 16 is a cross sectional view of a third modification of the batterycase lid.

FIG. 17 is a cross sectional view of a fourth modification of thebattery case lid.

FIG. 18 is a cross sectional view of a fifth modification of the batterycase lid.

EMBODIMENT FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described in detail withreference to the drawings. First, the configuration of a battery caselid 1 will be described. As shown in FIG. 1, the battery case lid 1according to the present embodiment is a metal plate member that closesan opening Q of a battery case P for use in a secondary battery. Thebattery case P is filled with an electrolyte. The battery case P and thebattery case lid 1 are welded to each other to hermetically seal thebattery case P. In the description, the directions of up, down, right,left, front, and rear are as indicated by the arrows in FIG. 1.

As shown in FIG. 1, the battery case lid 1 includes a substrate section2, a folded part 3, and an explosion-proof valve 4. The battery case lid1 is formed by working a single aluminum alloy plate. The material ofthe battery case lid 1 is not limited to an aluminum alloy and may beother metals such as copper and iron.

The substrate section 2 is a flat plate member that closes the opening Qof the battery case P. In plan view, the substrate section 2 isrectangular in shape to fit the opening Q. The substrate section 2 has auniform thickness of, for example, 1.2 mm-3.0 mm. The shape of thesubstrate section 2 may be altered properly according to the shape ofthe opening Q. The substrate section 2 has a through hole for insertionof an electrode, an injection hole for injection of an electrolyte, andthe like hole, illustration of such holes being omitted.

As shown in FIGS. 2A and 2B, the folded part 3 is a part folded in threering-shaped metal-plate layers formed in the substrate section 2. Thefolded part 3 extends around the entire outer periphery of theexplosion-proof valve 4. The folded part 3 is increased in thicknesscompared to the substrate section 2 in order to protect (reinforce) theperiphery of the explosion-proof valve 4. The metal-plate layers formingthe folded part 3 are gradually decreased in thickness from the bottomto the top layer. The folded part 3 may have any thickness as long as itis thicker than the substrate section 2. According to the presentembodiment, the folded part 3 has a thickness about double the thicknessof the substrate section 2. The folded part 3 is formed in folding stepsto be described later.

As shown in FIGS. 2A and 2B and FIG. 3, the explosion-proof valve 4 islocated inside the folded part 3. The explosion-proof valve 4 is a valvedesigned to rupture to release the internal pressure from inside thebattery case P when the internal pressure exceeds a predetermined value.The explosion-proof valve 4 includes a central elevation 42, a firstridge 43, a second ridge 44, a first groove 45, and a second groove 46,all formed in a reduced thickness section 41.

The reduced thickness section 41 has a circular shape in plan view andis decreased in thickness compared to the substrate section 2. Thethickness of the reduced thickness section 41 is set properly accordingto the operational pressure at which the internal pressure is released,and is set, for example, to 0.1 mm-1.5 mm. The reduced thickness section41 is positioned approximately at the same height as the upper face ofthe substrate section 2.

The central elevation 42 is formed in the center of the reducedthickness section 41. The central elevation 42 has a circular shape inplan view and is raised upward from the reduced thickness section 41.The central elevation 42, which is formed in the center of theexplosion-proof valve 4 and raised upward, is more susceptible to theinternal pressure from inside the battery case P.

The first ridge 43 and the second ridge 44 each have a ring shapeextending along the periphery of the central elevation 42 and areprotruded upward from the reduced thickness section 41. The first ridge43 and the second ridge 44 are concentric with the explosion-proof valve4. The first ridge 43 is located inside the second ridge 44. The firstridge 43 and the second ridge 44 are portions formed for removing excessmetal (spread away metal) produced in the formation of the first groove45 and the second groove 46, so that the first ridge 43 and the secondridge 44 are located along the first groove 45 and the second groove 46.

According to the present embodiment, two ridges are formed. However, noridges may be formed. Further, since two grooves (the first groove 45and the second groove 46) are formed according to the presentembodiment, there may be another ridge outside the second groove 46.Moreover, the ridges, which are protruded upward according to thepresent embodiment, may be protruded downward instead.

As shown in FIGS. 2A and 2B, the first groove 45 and the second groove46 are ring-shaped grooves formed in the upper face of the reducedthickness section 41. The first groove 45 and the second groove 46 areconcentric with the explosion-proof valve 4. The first groove 45 islocated between the first ridge 43 and the second ridge 44, and thesecond groove 46 is located outside the second ridge 44.

As shown in FIG. 3, each of the first groove 45 and the second groove 46is V-shaped in cross section. In other words, each of the first groove45 and the second groove 46 is open up with its width graduallyincreasing from bottom to top. The portions where the first groove 45and the second groove 46 are formed are even further reduced inthickness compared to the remainder of the reduced thickness section 41.Thus, at least one of the first groove 45 and the second groove 46easily ruptures when the internal pressure from inside the battery caseP exceeds the predetermined value. The thickness of the portions of thereduced thickness section 41 where the first groove 45 and the secondgroove 46 are formed, which are thicknesses M1 and M2, respectively, areset properly according to the operational pressure to be set.

In addition, the first groove 45 and the second groove 46 may beidentical or different from each other in both cross section and planview. Further, one of the first groove 45 and the second groove 46 maybe deeper than the other. Further, the first groove 45 and the secondgroove 46 may have another shape than the V-shape in cross section.Moreover, one or three or more grooves may be formed instead of two asin the present embodiment.

Next, a manufacturing method for the battery case lid 1 will bedescribed.

As shown in FIG. 4, the manufacturing method for the battery case lid 1according to the present embodiment includes an extending step, a firstcorrecting step, a second correcting step, a bending step, a preliminaryfolding step, a primary folding step, and a groove forming step.

In the manufacturing method for the battery case lid 1 according to thepresent embodiment, a single metal plate K is worked to form the batterycase lid 1, as shown in FIG. 5A. According to the present embodiment,the metal plate K is an aluminum alloy plate. The metal plate K has athickness t set properly to, for example, 1.0 mm-3.0 mm according to theapplication of the secondary battery. The thickness t of the metal plateK is equal to the thickness of the substrate section 2 of the batterycase lid 1.

<Extending Step>

In the extending step, as shown in FIGS. 5A-5D, by working the metalplate K, a metal plate workpiece K1 is formed with the substrate section2, a cylindrical wall portion 31, and the reduced thickness section 41.In the extending step, there are used an extending die 101 as a lowerdie and an extending punch 102 as an upper die.

As shown in FIG. 5A, the extending die 101 is a die including a flatupper face 103 and a recess 104 formed therein. The recess 104 has acircular shape in plan view, and has a bottom face 104 a and a side face104 b upstanding relative to the bottom face 104 a. The side face 104 bis cylindrical. An internal corner portion 104 c between the bottom face104 a and the side face 104 b has a peripheral portion arc-shaped incross section. An external corner portion 104 d between the side face104 b and the upper face 103 has a peripheral portion arc-shaped incross section. The recess 104 has a depth e (a distance from the upperface 103 to a place of the bottom face 104 a where the recess 104 has agreatest depth) which is slightly greater than the thickness t of themetal plate K according to the present embodiment. The bottom face 104 aof the recess 104 may be flat, but according to the present embodiment,it is constituted of a spherical face which is slightly downwardlyconvex. The spherical face of the bottom face 104 a has a curvatureradius R1 of, for example, 1000 mm-1500 mm.

The extending punch 102 is a die that moves up and down relative to theextending die 101 and has a body portion 105 and a protrusion 106 formedon the body portion 105. The extending punch 102 moves up and downcoaxially with the recess 104 of the extending die 101. The body portion105 has a columnar shape having the pressing face 102 a on the lowerend. The pressing face 102 a may be flat, but according to the presentembodiment, it is constituted of a spherical face which is slightlydownwardly convex. The spherical face of the pressing face 102 a has acurvature radius R2 of, for example, 1000 mm-1500 mm. Preferably, thecurvature radius R2 of the spherical face of the pressing face 102 a isequal to or smaller than the curvature radius R1 of the spherical faceof the bottom face 104 a of the recess 104.

The protrusion 106, at the end of the body portion 105, is slightlyprotruded laterally from a side face 105 a of the body portion 105 alongthe entire periphery thereof. The protrusion 106 has a semi-circularshape protruding outwardly in cross section. According to the presentembodiment, the arc in the cross section of the protrusion 106, the arcin the cross section of the internal corner portion 104 c, and the arcin the cross section of the external corner portion 104 d have the samecurvature radius.

As shown in FIG. 5A, a clearance c is formed between the side face 104 bof the recess 104 and the outer periphery of the protrusion 106 (in thepresent embodiment, the outer periphery including an apex line 106 a).According to the present embodiment, the clearance c is smaller than thethickness t of the metal plate K.

In the extending step, as shown in FIGS. 5A-5D, the extending punch 102is moved down from one face Ka side of the metal plate K that ispositioned over the extending die 101. Then, when the metal plateworkpiece K1 is deformed to come into contact with the bottom face 104 aas shown in FIG. 5B, the extending punch 102 is moved further down asshown in FIGS. 5C and 5D. According to the present embodiment, in orderto form the reduced thickness section 41 to a thickness of, for example,0.3 mm, the extending punch 102 is descended to a position wherein thedistance between the pressing face 102 a and the bottom face 104 a is0.3 mm.

When the extending punch 102 is descended, pressure is applied to extenda portion of the metal plate workpiece K1 that is sandwiched between thepressing face 102 a and the bottom face 104 a, while gradually reducingthe thickness of that portion of the metal plate workpiece K1 andspreading the metal away radially from the center of the extending punch102. The spread away metal comes into contact with the side face 104 bof the recess 104, and changes its direction to flow vertically upward.Accordingly, the metal plate workpiece K1 is made to have the substratesection 2, the cylindrical wall portion 31 formed as a result of thedepression of the substrate section 2, and the reduced thickness section41 formed at a bottom face of the cylindrical wall portion 31. Thesubstrate section 2 is formed extending over the extending die 101 atthe formation of the cylindrical wall portion 31.

The cylindrical wall portion 31 has a height h which, according to thepresent embodiment, is, for example, about 10 mm. The height h isgreater than the depth e of the recess 104 of the extending die 101. Inaddition, a pushed-in distance f of the extending punch 102 (a distancebetween the position where the pressing face 102 a contacts the metalplate K and the lowest position of the pressing face 102 a) is greaterthan the depth e of the recess 104 of the extending die 101.

If the clearance c between the side face 104 b of the recess 104 and theextending punch 102 is equal to or greater than the thickness t of themetal plate K, strains (creases) occur in a portion of the metal plateworkpiece K1 which is not sandwiched between the extending die 101 andthe extending punch 102, i.e., in a portion forming the cylindrical wallportion 31, during the operation of extending the metal plate workpieceK1 by applying pressure to form the reduced thickness section 41, anoperation subsequent to drawing the metal plate workpiece K1 into theextending die 101. The strains adversely affect working stability in thenext operation and thereafter.

According to the present embodiment, the clearance c between the sideface 104 b of the recess 104 and the extending punch 102 is set to besmaller than the thickness t of the metal plate K. Because a portion ofthe metal plate workpiece K1 that first starts moving in the extendingstep is the portion of the metal plate workpiece K1 sandwiched betweenthe extending die 101 and the extending punch 102, that is, the portionforming the reduced thickness section 41, the above configuration canserve to reduce strains in the portion of the metal plate workpiece K1which is not sandwiched between the extending die 101 and the extendingpunch 102. The reduction of strains improves the working stability inthe next operation and thereafter, and consequently improves the qualityof the product. Here, if the relationship between the clearance c andthe thickness t is 0.8 t≈c, the height h of the cylindrical wall portion31 is about 10 mm.

FIG. 6 shows the metal plate workpiece having gone through the extendingstep. As shown in FIG. 6, the substrate section 2 of the metal plateworkpiece K1 is tilted diagonally upward relative to the horizontalplane due to spring back. The cylindrical wall portion 31 has arelatively thick base portion 32 and a tapered portion 33. The taperedportion 33 gradually decreases in thickness from an end portion of thebase portion 32 to the reduced thickness section 41. An internal cornerportion 33 a, formed between the tapered portion 33 and the reducedthickness section 41, has a peripheral portion arc-shaped in crosssection. The tapered portion 33 has a taper wherein the thicknessreduces gradually from top to bottom because the metal flowing out ofthe recess 104 gradually decreases in amount with the progress of theextending step of forming the reduced thickness section 41.

The reduced thickness section 41 is slightly convex in the pushingdirection of the extending punch 102 (in FIG. 6, downwardly).

<First Correcting Step>

In the first correcting step, as shown in FIGS. 7A and 7B, the reducedthickness section 41 of the metal plate workpiece K1 is corrected to beflat. In the first correcting step, there are used a first correctingdie 111 as a lower die and a first correcting punch 112 as an upper die.

The first correcting die 111 is a die having a recess 114 in an upperface 113. The recess 114 is circular in plan view and has a diameterslightly greater than the outer diameter of the reduced thicknesssection 41. The recess 114 has a depth smaller than the height of thecylindrical wall portion 31. The recess 114 has a flat bottom face.

The first correcting punch 112 is a die having a columnar shape. Thefirst correcting punch 112 has an outer diameter slightly smaller thanthe inner diameter of the cylindrical wall portion 31. The firstcorrecting punch 112 moves up and down coaxially with the recess 114 ofthe first correcting die 111. The first correcting punch 112 has a flatpressing face 112 a.

In the first correcting step, the metal plate workpiece K1 is placedwith the reduced thickness section 41 within the recess 114 and with oneface Ka of the metal plate workpiece K1 up. In this state, the firstcorrecting punch 112 is pressed against the reduced thickness section 41so that the reduced thickness section 41 is corrected to be flat asshown in FIG. 7B.

<Second Correcting Step>

In the second correcting step, as shown in FIGS. 8A and 8B, thecylindrical wall portion 31 is corrected to be perpendicular to thesubstrate section 2 of the metal plate workpiece K1. In the secondcorrecting step, there are used a second correcting die 121 and a secondcorrecting punch 122, as shown in FIG. 8A.

The second correcting die 121 is a die having a protruded portion 124 ona flat upper face 123. The protruded portion 124 has a columnar shapeand has an outer diameter slightly smaller than the inner diameter ofthe cylindrical wall portion 31. The protruded portion 124 has a flatupper face. The protruded portion 124 has a height approximately equalto that of the cylindrical wall portion 31.

The second correcting punch 122 is a die having a cylindrical shape. Thesecond correcting punch 122 has a hollow inside with an inner diameterthereof greater than the outer diameter of the cylindrical wall portion31. The second correcting punch 122 moves up and down coaxially with theprotruded portion 124 of the second correcting die 121.

In the second correcting step, the metal plate workpiece K1 is placedupside down over the second correcting die 121 with the other face Kb upand with the reduced thickness section 41 positioned over the protrudedportion 124. Then, the second correcting punch 122 is pressed againstthe metal plate workpiece K1. In the second correcting step, thecylindrical wall portion 31 is corrected to be perpendicular to thesubstrate section 2 of the metal plate workpiece K1, as shown in FIG.8B.

<Bending Step>

In the bending step, as shown in FIGS. 9A and 9B, the reduced thicknesssection 41 of the metal plate workpiece K1 is bent back. In the bendingstep, there are used a bending die 131 as a lower die and a bendingpunch 132.

The bending die 131 is a die having a protruded portion 134 on a flatupper face 133. The protruded portion 134 has an approximately columnarshape. The protruded portion 134 has an outer diameter smaller than theinner diameter of the cylindrical wall portion 31. The protruded portion134 has a flat upper face 134 a. The protruded portion 134 has a heightabout one fourth of the height of the cylindrical wall portion 31.

The bending punch 132 is a die having an approximately columnar shape.The bending punch 132 has a body portion 135 and a radially diminishingportion 136 extending from the bottom of the body portion 135. Theradially diminishing portion 136 diminishes gradually in radius from topto bottom. The bending punch 132 moves up and down coaxially with theprotruded portion 134 of the bending die 131. The body portion 135 hasan outer diameter smaller than the inner diameter of the cylindricalwall portion 31. The radially diminishing portion 136 has a pressingface 136 a with a diameter smaller than the inner diameter of thereduced thickness section 41.

In the bending step, the metal plate K 1 is placed on the bending die131 with the other face Kb of the metal plate workpiece K1 up and withthe reduced thickness section 41 positioned over the protruded portion134. Then, the bending punch 132 is pressed downward against the reducedthickness section 41 to the extent that the reduced thickness section 41contacts the protruded portion 134, as shown in FIG. 9B. As a result,the height of the reduced thickness section 41 is reduced toapproximately two thirds of the height thereof before the bending step,the tapered portion 33 of the cylindrical wall portion 31 is turned backinside, and also the base portion 32 is moved slightly outward. Aportion bent by the turning back of the tapered portion 33 is referredto as a bent portion 34. The bent portion 34 is formed in a ring shapein plan view.

<Preliminary Folding Step>

In the preliminary folding step, as shown in FIGS. 10A and 10B, the bentportion 34 of the metal plate workpiece K1 is pushed open outward to befolded halfway to the direction toward the substrate section 2. In otherwords, the preliminary folding step is a preliminary step to ensurefolding of the cylindrical wall portion 31. In the preliminary foldingstep, there are used a preliminary folding die 141 as a lower die, apreliminary folding punch 142 as an upper die, and a holding means G.

The preliminary folding die 141 is a die having an upper face 143. Theholding means G is a member for holding the metal plate workpiece K1against the upper face 143.

The preliminary folding punch 142 is a die having a columnar shape, andmoves up and down relative to the preliminary folding die 141. Thepreliminary folding punch 142 has a body portion 144 and a bottomportion 145 extending from the bottom of the body portion 144. The bodyportion 144 has a diameter greater than the outer diameter of the baseportion 32 of the cylindrical wall portion 31. The bottom portion 145has the shape of a truncated cone with a downwardly reducing diameter.The bottom portion 145 has an upper end 145 a with a diameter slightlygreater than the outer diameter of the base portion 32. The bottomportion 145 has a pressing face 145 b with a diameter approximatelyequal to the diameter of the reduced thickness section 41.

In the preliminary folding step, the upper face 143 and the holdingmeans G are used to immobilize the metal plate workpiece K1 placed onthe preliminary folding die 141 with the other face Kb of the metalplate workpiece K1 up. Then, the reduced thickness section 41 is pusheddownward with the pressing face 145 b while the bent portion 34 is beingpushed open outward with a taper 145 c of the bottom portion 145 of thepreliminary folding punch 142. This brings the base portion 32 and thetapered portion 33 into a face-to-face contact, and reduces the heightbetween the substrate section 2 and the top of the bent portion 34 tohalf the height before the preliminary folding step, as shown in FIG.10B. In addition, the lower surface of the reduced thickness section 41is brought to approximately the same height as the upper face of thesubstrate section 2.

<Primary Folding Step>

In a primary folding step, as shown in FIGS. 11A and 11B, the bentportion 34 of the metal plate workpiece K1 is pushed open outward to befolded fully onto the substrate section 2. In the primary folding step,there are used a primary folding die 151 as a lower die, a primaryfolding punch 152 as an upper die, and the holding means G.

The primary folding die 151 is a die having a flat upper face 153. Theholding means G is a member for holding the metal plate workpiece K1against the upper face 153.

The primary folding punch 152 is a die having an approximately columnarshape. The primary folding punch 152 moves up and down relative to theprimary folding die 151. The primary folding punch 152 has a bodyportion 154 and a protruded portion 155 extending from a lower surface154 a of the body portion 154. The body portion 154 has an outerdiameter greater than the outer diameter of the base portion 32. Thelower surface 154 a of the body portion 154 is flat.

The protruded portion 155 is protruded downward from the lower surface154 a of the body portion 154, and has an approximately columnar shape.The protruded portion 155 has a flat pressing face 155 a. The protrudedportion 155 has a bottom end with a peripheral portion arc-shaped incross section. The protruded portion 155 has a height approximatelyequal to the thickness of the substrate section 2.

In the primary folding step, the upper face 153 and the holding means Gare used to immobilize the metal plate workpiece K1 placed on theprimary folding die 151 with the other face Kb of the metal plateworkpiece K1 up. Then, the reduced thickness section 41 is pusheddownward with the pressing face 155 a of the protruded portion 155 whilethe bent portion 34 is being pushed open outward with the lower surface154 a of the primary folding punch 152. This brings the base portion 32and the substrate section 2 into a face-to-face contact. In addition,the upper face of the reduced thickness section 41 is brought toapproximately the same height as the upper face of the substrate section2. In the primary folding step, the folded part 3 extends around thereduced thickness section 41.

<Groove Forming Step>

In the groove forming step, as shown in FIGS. 12A and 12B, grooves andthe like are formed at the reduced thickness section 41. In the grooveforming step, there are used a groove forming die 161 as a lower die anda groove forming punch 162 as an upper die.

The groove forming die 161 is a die having a protruded portion 164 on aflat upper face 163. The protruded portion 164 has an approximatelycolumnar shape. The protruded portion 164 has an upper face formed withprojections and recesses for forming the central elevation 42, the firstridge 43, the second ridge 44, the first groove 45, and the secondgroove 46, as shown in FIGS. 2A and 2B.

The groove forming punch 162 is a die that moves up and down coaxiallywith the protruded portion 164 of the groove forming die 161. The grooveforming punch 162 has a pressing face 162 a formed with projections andrecesses for forming the central elevation 42, the first ridge 43, thesecond ridge 44, the first groove 45, and the second groove 46, as shownin FIGS. 2A and 2B.

In the groove forming step, the reduced thickness section 41 is placedover the protruded portion 164 with the other face Kb of the metal plateworkpiece K1 up. Then, the groove forming punch 162 is pressed againstthe groove forming die 161 to form the reduced thickness section 4 withthe central elevation 42, the first ridge 43, the second ridge 44, thefirst groove 45, and the second groove 46. Thus, the battery case lid 1is formed through the above steps.

According to the manufacturing method for the battery case lid 1described above, since the metal plate K is pushed by the extendingpunch 102 while being kept unrestrained, a reduction is made in anamount of strains introduced at the formation of the reduced thicknesssection, and as a result, work hardening of the metal plate workpiece K1is inhibited. This prevents crack of the reduced thickness section 41 atthe formation of the first groove 45 and the second groove 46 in thereduced thickness section 41. Further, the reduced thickness section 41is formed merely by pushing the extending punch 102 against the metalplate K, which facilitates the operations performed for the formation ofthe reduced thickness section 41. Moreover, since work hardening of themetal plate workpiece K1 is inhibited, the annealing process may beomitted, reducing the number of operational steps.

Further, in the extending step, while the thickness of the reducedthickness section 41 is gradually being reduced, the metal spread awayby the bottom face 104 a and the extending punch 102 a is guided to flowthrough the clearance c between the side face 104 b of the recess 104and the extending punch 102 to the outside of the clearance c andfurther vertically. This enables the cylindrical wall portion 31 to beformed approximately perpendicularly to the reduced thickness section41. Further, according to the present embodiment, since the internalcorner portion 104 c of the recess 104 and the protrusion 106 of theextending punch 102 each have an arc shape, that metal smoothly flowsout.

Further, the clearance c between the side face 104 b of the recess 104of the extending die 101 and the extending punch 102 is smaller than thethickness t of the metal plate K. Because the portion of the metal plateworkpiece K1 which first starts moving in the extending step is theportion sandwiched between the extending die 101 and the extending punch102, the above configuration can serve to reduce strains in the portionof the metal plate workpiece K1 which is not sandwiched between theextending die 101 and the extending punch 102, i.e., in the portionforming the cylindrical wall portion 31. The reduction of strainsimproves the working stability.

Further, since the pressing face 102 a of the extending punch 102 is aspherical face which is convex in the pressing direction of theextending punch 102, the metal plate workpiece K1 is pressed to beextended radially from the center of the metal plate workpiece K1, sothat the reduced thickness section 41 can have a uniform thickness.

Moreover, since the side face 105 a of the extending punch 102 has a topsection with the lateral protrusion 106 formed thereon, friction betweenthe side face 105 a of the extending punch 102 and the metal plateworkpiece K1 at the working can be avoided. This prevents deformation ofthe metal plate workpiece K1 due to friction resistance relative to theextending punch 102.

Furthermore, since performing the bending step enables reduction in theheight of the cylindrical wall portion 31, the height of the batterycase lid 1 can be reduced. Further, since performing the folding stepsenables formation of the folded part 3, the height of the cylindricalwall portion 31 can be further reduced.

Moreover, since the explosion-proof valve 4 is surrounded by the foldedpart 3 formed thick by folding the metal plate K, the peripheral area ofthe explosion-proof valve 4 can be protected. Further, since the foldedpart 3 can block heat produced by welding or the like, a reduction canbe made in an amount of heat entering the explosion-proof valve 4.Further, since the folded part 3 can be formed by merely folding themetal plate workpiece K1, the manufacturing process can be facilitated.

Further, the grooves of the battery case lid 1 (the first groove 45 andthe second groove 46) each have a width gradually increasing in theapplication direction of the internal pressure from inside of thebattery case P (in the present embodiment, upwardly). Accordingly, whenthe internal pressure acts on the explosion-proof valve 4, the groovestend to be deformed in such a direction as to open across their widths.This ensures the rupture of the grooves. Further, since the ridges (thefirst ridge 43 and the second ridge 44) are formed in the battery caselid 1, the excess metal produced in the formation of the grooves can beeffectively removed to prevent creation of strains.

The embodiment of the present invention has been described. However,design modifications and variations can be made properly to theembodiment of the present invention without departing from the scope ofthe invention. FIG. 13 is an enlarged cross sectional view showing amodification of the grooves of the explosion-proof valve. As shown inFIG. 13, a reduced thickness section 41A has a first groove 45A and asecond groove 46A. Each of the first groove 45A and the second groove46A includes an enlarged portion 51 having a greater width and anarrowed portion 52 formed in the bottom face of the enlarged portion51. The groove may have a two-staged depth as described above.

Further, according to the present embodiment, the explosion-proof valve4 has a circular shape in plan view. However, the shape of theexplosion-proof valve 4 is not limited thereto and may be oval orellipse. In addition, the dimensions in the embodiment are given merelyby way of example and not meant to limit the invention.

Further, according to the present embodiment, as shown in FIG. 4, thegroove forming step follows the primary folding step. However, there isno limitation thereto, and the groove forming step may follow any one ofthe extending step, the first correcting step, the second correctingstep, the bending step, and the preliminary folding step.

FIG. 14 is a cross sectional view of a first modification of the batterycase lid. In a battery case lid 1A as the first modification shown inFIG. 14, the extending step is followed by the groove forming step inwhich the central elevation 42, the first ridge 43, the second ridge 44,the first groove 45, and the second groove 46 are formed in the reducedthickness section 41.

FIG. 15 is a cross sectional view of a second modification of thebattery case lid. In a battery case lid 1B as the second modification ofFIG. 15, the first correcting step is followed by the groove formingstep in which the central elevation 42, the first ridge 43, the secondridge 44, the first groove 45, and the second groove 46 are formed inthe reduced thickness section 41.

FIG. 16 is a cross sectional view of a third modification of the batterycase lid. In a battery case lid 1C as the third embodiment of FIG. 16,the second correcting step is followed by the groove forming step inwhich the central elevation 42, the first ridge 43, the second ridge 44,the first groove 45, and the second groove 46 are formed in the reducedthickness section 41.

FIG. 17 is a cross sectional view of a fourth modification of thebattery case lid. In a battery case lid 1D as the fourth modification ofFIG. 17, the bending step is followed by the groove forming step inwhich the central elevation 42, the first ridge 43, the second ridge 44,the first groove 45, and the second groove 46 are formed in the reducedthickness section 41.

FIG. 18 is a cross sectional view of a fifth modification of the batterycase lid. In a battery case lid 1E as the fifth modification of FIG. 18,the preliminary folding step is followed by the groove forming step inwhich the central elevation 42, the first ridge 43, the second ridge 44,the first groove 45, and the second groove 46 are formed in the reducedthickness section 41.

According to the first modification 1A to the fourth modification 1D,the cylindrical wall portion 31 is made thicker than the reducedthickness section 41 to protect the reduced thickness section 41.According to the fifth modification 1E, performing the preliminaryfolding step enables the base portion 32, the tapered portion 33, andthe bent portion 34 to lie on one another, creating a portion thickerthan the reduced thickness section 41 to protect the reduced thicknesssection 41.

Further, according to the present embodiment, the extending step isperformed to form the reduced thickness section 41. However, there is nolimitation thereto, and the extending step may be replaced with, forexample, press-working or deep drawing, spinning. Or, other working thatenables the metal plate to take the shape of a cylinder with a bottommay be performed. Further, these workings may be followed by theformation of the reduced thickness section, which is then subjected toin the groove forming step to have a recess or groove formed in thereduced thickness section, thereby forming the battery case lid.

Further, according to the present embodiment, as shown in FIGS. 2A and2B, the folded part 3 is formed on the outer side of the battery caselid (on the face opposite to the face to which the internal pressure isapplied) thereof, but may be formed on the inner side instead.

DESCRIPTION OF REFERENCE CHARACTERS

-   1 battery case lid-   2 substrate section-   3 folded part-   4 explosion-proof valve-   31 cylindrical wall portion-   32 base portion-   33 tapered portion-   34 bent portion-   41 reduced thickness section-   42 central elevation-   43 first ridge-   44 second ridge-   45 first groove (groove)-   46 second groove (groove)-   101 extending die-   102 extending punch-   104 recess-   104 a bottom face-   104 b side face-   131 bending die-   132 bending punch-   141 preliminary folding die-   142 preliminary folding punch-   151 primary folding die-   152 primary folding punch-   161 groove forming die-   162 groove forming punch-   K metal plate-   K1 metal plate workpiece-   Ka one face-   Kb the other face-   c clearance-   e depth of recess-   t thickness of metal plate

1. A battery case lid formed by working a metal plate, the battery caselid comprising: a substrate section; and an explosion-proof valve formedin the substrate section, wherein the explosion-proof valve has areduced thickness section thinner than the substrate section, and thereduced thickness section is formed by extending the metal plate byapplying pressure with the metal plate kept unrestrained.
 2. The batterycase lid of claim 1, wherein the reduced thickness section has a groove.3. The battery case lid of claim 1, wherein the explosion-proof valve issurrounded by a cylindrical wall portion thicker than the reducedthickness section.
 4. The battery case lid of claim 1, wherein theexplosion-proof valve is surrounded by a folded part thicker than thereduced thickness section, the folded part being formed by folding themetal plate.
 5. A manufacturing method for a battery case lid includingan explosion-proof valve, the manufacturing method comprising: anextending step of pushing an extending punch from a first side of ametal plate disposed over an extending die having a recess, to form areduced thickness section constituting the explosion-proof valve,wherein the recess has an inter diameter greater than an outer diameterof the extending punch, and wherein in the extending step, the extendingpunch is pushed while the metal plate is kept unrestrained, and themetal plate is pressed to be extended between a bottom face of therecess and a pressing face of the extending punch, to form the reducedthickness section.
 6. The manufacturing method for the battery case lidof claim 5, further comprising a groove forming step of forming a groovein the reduced thickness section.
 7. The manufacturing method for thebattery case lid of claim 5, wherein in the extending step, acylindrical wall portion is formed around the reduced thickness section,the cylindrical wall portion having a height greater than a depth of therecess of the extending die.
 8. The manufacturing method for the batterycase lid of claim 5, wherein the extending step is followed by a bendingstep in which the reduced thickness section is pressed by a bendingpunch smaller in outer diameter than the reduced thickness section froma second side of the metal plate, to bend a cylindrical wall portionformed around the reduced thickness section.
 9. The manufacturing methodfor the battery case lid of claim 8, wherein the bending step isfollowed by a folding step in which a bent portion formed by bending thecylindrical wall portion in the bending step is pushed by a foldingpunch from the second side of the metal plate, to fold the bent portion.10. The manufacturing method for the battery case lid of claim 5,wherein a clearance between a side face of the recess of the extendingdie and the extending punch is set to be smaller than a thickness of themetal plate.
 11. The manufacturing method for the battery case lid ofclaim 5, wherein the pressing face of the extending punch has aspherical face which is convex in a pressing direction of the extendingpunch.
 12. The manufacturing method for the battery case lid of claim 5,wherein a side face of the extending punch has a top section with alateral protrusion formed thereon.
 13. The manufacturing method for thebattery case lid of claim 5, wherein the extending step is followed by acorrecting step in which the reduced thickness section is corrected tobe flat.
 14. A battery case lid formed by working a metal plate, thebattery case lid comprising: a substrate section; and an explosion-proofvalve formed in the substrate section, wherein the explosion-proof valvehas a reduced thickness section thinner than the substrate section, andthe explosion-proof valve is surrounded by a cylindrical wall portionthicker than the reduced thickness section.
 15. A battery case lidformed by working a metal plate, the battery case lid comprising: asubstrate section; and an explosion-proof valve formed in the substratesection, wherein the explosion-proof valve has a reduced thicknesssection thinner than the substrate section, and the explosion-proofvalve is surrounded by a folded part thicker than the reduced thicknesssection, the folded part being formed by folding the metal plate. 16.The battery case lid of claim 14, wherein the reduced thickness sectionhas a groove.
 17. The battery case lid of claim 15, wherein the reducedthickness section has a groove.